A global perspective of the Trigoniida (Bivalvia: Palaeoheterodonta), with a focus on their Mesozoic and Cenozoic representatives

The Trigoniida Dall, 1889 are a globally important group of dominantly marine Bivalvia, which had their acme during the Mesozoic, when they played an influential role in shallow marine communities. The anticipated revision of the bivalve volumes of the ‘Treatise’ has fostered the authors’ attempt on a global genus-level systematic arrangement for the Mesozoic and Cenozoic Trigoniida. Currently, classification is not based on rigid phylogenetic analysis, but on qualitatively defined apomorphies. Nevertheless, it allows for identification of several palaeogeographically and stratigraphically well-constrained clades. Other clades, in particular the Myophorellidae and Pterotrigoniidae are considerably less resolved and their subdivision is controversially discussed, mainly due to conflicting genus-level concepts. A circum-Pacific origin seems obvious for most of the Triassic clades of the Trigoniida. Subsequently, the break-up of Pangaea and later Gondwana created space for global expansion across shelf seas. At the same time, isolation forced more localized radiations of several groups. We present chronostratigraphic range charts for the genera currently recognized as valid, supplemented by palaeogeographic distribution maps for selected taxa. We further exemplarily discuss difficulties of generic assignment in the Myophorellidae. Finally, we propose a revised classification for the Trigoniida that is put up for discussion.